1. Field of the Invention
This invention relates to a catalyst for purifying an exhaust gas discharged from an internal engine for automobiles or the like.
2. Description of the Related Art
There have hitherto been used catalysts for the purification of exhaust gas using zeolite as disclosed in JP-A-1-127044. This catalyst for the purification of exhaust gas has a structure that a first catalyst layer inclusive of a noble metal ingredient generating oxidation reaction is coated onto a monolith carrier and then a second catalyst layer obtained by ion-exchanging zeolite with Cu is formed on the first catalyst layer. When such a catalyst is used in the purification of the exhaust gas, hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxides (NOx) are efficiently purified even in an exhaust gas atmosphere having an oxygen concentration higher than theoretical value (lean-burn atmosphere) to improve the catalytic purification performance.
In the catalyst having the above structure, however, the performances after an engine aging treatment are considerably degraded as compared with those of a catalyst having no oxidation catalytic layer as an inner layer and can not provide satisfactory purification activity.